The present invention relates to an apparatus for forming microscopic recesses on a circumferential surface that defines a cylindrical bore in a workpiece and comes into sliding contact with a counterpart, and a method of forming microscopic recesses on the circumferential surface thereof by using the apparatus. More specifically, the present invention relates to an apparatus for forming microscopic recesses as oil retention portions on the circumferential surface, for instance, a cylinder bore surface of a cylinder block of an engine for automobiles, an outer circumferential surface of a cam shaft of the engine and the like, in order to reduce friction on the cylinder bore surface, and relates to a method of forming microscopic recesses on the circumferential surface by using the apparatus.
Conventionally, upon forming microscopic recesses on a circumferential surface that defines a cylindrical bore in a workpiece, the circumferential surface is subjected to shot blasting. Upon shot blasting, a masking sheet with through-holes having a predetermined shape is attached to the circumferential surface, and then small-diameter shots, such as ceramic balls, are blasted with compressed air against the circumferential surface. As a result, microscopic recesses are formed on a part of the circumferential surface which is exposed outside through the through-holes. Subsequently, the masking sheet is taken off, and the circumferential surface is subjected to cleaning and honing to thereby remove protrudent peripheral portions around the microscopic recesses which are formed upon shot blasting. Japanese Patent Application First Publication No. 2002-307310 describes such a masking and blasting method as explained above.